As is known, D MOS n-channel power transistors are currently extensively used as high-side drivers in bridge circuits to drive inductive loads. A DMOS n-channel power transistor is a double-diffused N-MOS transistor with a very short channel and a drift region between its channel and drain. See, for example, IEEE Transactions on Electron Devices, Vol. ED-19, No. 1, Jan. 1972.
Such bridge circuits have to be supplied electrically, in addition to a supply voltage Vcc, with a higher voltage Vcp than the supply voltage which is applied to the gate electrode on such power transistors in use as high-side drivers. This additional voltage value is specially needed under those conditions of operation when a power transistor delivers maximum current with a minimum voltage drop between its drain and source electrodes.
Furthermore, that voltage value is supplied to that transistor in the bridge circuit by means of a purposely provided turn-on circuit.
The voltage Vcp may be supplied by a feeder; however, where such a feeder is not available, said voltage value would be generated at one end of a so-called bootstrap capacitor Cp arranged to function as a charge storage. To that one end of the capacitor Cp, each turn-on circuit for each power transistor employed as a high-side driver in the bridge circuit is connected. The capacitor Cp is re-charged periodically by means of a re-charge circuit so as to keep the voltage value Vcp unchanged.
It is a current practice in the art to use, for each power transistor, a respective turn-on circuit which is input connected and powered directly from said capacitor, to which a plurality of power transistors from several possible bridge circuits may be connected.
That prior approach has drawbacks deriving from that, on changing over the power transistor from the off state to the on state, the achievement of a correct supply voltage value on the gates of such transistors is entirely demanded to said capacitor, which will exhibit a high absorption of charges. As a result, the use of large size capacitors and of a re-charge circuit of correspondingly large size becomes mandatory.